This invention relates to an optical apparatus suitable for transmission/reception of e.g., signal light for optical communication.
In these years, in keeping up with information diversification, thus is with the tendency towards multi-media, development of a small-sized, high-performance low-cost communication apparatus has become a desideratum. The optical communication by a two-core optical fiber, having a glass optical fiber for transmission and a glass optical fiber for reception, has already been put to practical use because it permits high transmission rate and long-distance transmission and also because it is strong against electromagnetic noise. However, in the optical communication employing the two-core optical fiber, the optical fiber and the communication apparatus are both expensive, such that it is not used extensively in households and finding only limited practical application. Thus, the recent tendency is towards communication employing a sole inexpensive plastic optical fiber, such that preparations are being made for a communication environment by a uni-core optical fiber.
FIGS. 37 and 38 mainly show the schematic structure of optical components of a conventional optical communication apparatus employing a uni-core optical fiber. FIGS. 37 and 38 show an optical path L11 of the transmitting light along with the schematic structure of an optical system of the optical communication apparatus and an optical path L12 of the transmitting light along with the schematic structure of the optical system of the optical communication apparatus.
As shown in these figures, the optical system of the communication apparatus includes a light source 101 constructed by e.g., a semiconductor laser for radiating a transmitting laser light beam, and a collimator lens 102 for converting the light from the light source 101 into collimated light and for radiating a collimated light beam. The optical system also includes a polarization beam splitter 103 for reflecting the S-polarized component of the incident light substantially by total reflection and transmitting a P-polarized component of the incident light substantially by total transmission. The optical system also includes a coupling lens 104 for converging the transmitting light radiated from the polarization beam splitter 103 on an end face 105a of a uni-core optical fiber 105 and for radiating the received light radiated from the end face 105a of the optical fiber 105 as a collimated light beam. The optical system also includes a converging lens 106 for converging the collimated light beam radiated from the coupling lens 104, and a photodetector 107 for detecting the received light converged by the converging lens 106. The polarization beam splitter 103 includes an inclined surface 103a on the surface of which a dielectric multilayer film is formed for imparting a polarization beam splitter function, that is for reflecting an S-polarized light component of the incident light substantially by total reflection and for transmitting a P-polarized light component thereof substantially by total transmission. In the transmitting/reception device, the light source 101 and the polarization beam splitter 103 are arranged so that the plane of polarization of light radiated from the light source 101 to fall on the inclined surface 103a will the S-polarization plane. Thus, the light from the light source 101 (S-polarized light) undergoes substantially total reflection on the inclined surface 103a. 
In the above-described circuit apparatus, employing the polarization beam splitter 103, bidirectional optical communication, that is transmission and reception employing the laser light, becomes possible with the use of a sole device.
The optical communication in the transmission apparatus capable of bidirectional optical communication occurs as follows:
Referring first to FIG. 37, when light is transmitted from the circuit apparatus, the transmitting light is radiated from a light source 101 and collimated by the collimator lens 102 to fall on the polarization beam splitter 103. Since the light source 101 and the polarization beam splitter 103 are arranged relative to each other so that the plane of polarization of the light radiated from the light source 101 to fall on the inclined surface 103a will be the P-polarized light, the light radiated from the light source 101 is reflected substantially by total reflection by the inclined surface 103a. The light beam reflected by total reflection by the inclined surface 103a falls on the end face 105a of the optical fiber 105 via the coupling lens 104. The light incident on the optical fiber 105 is transmitted through the optical fiber 105 to the destination of communication as the signal light for communication.
Referring to FIG. 38, the signal light transmitted through the optical fiber 105 at the time of light reception by the communication apparatus is radiated from the end face 105aof the optical fiber 105. The light beam of the signal light radiated from the end face 105ais collimated by the coupling lens 104 of the communication apparatus so as to fall on the polarization beam splitter 103. The light beam incident on the polarization beam splitter 103 has a random plane of polarization (light of random polarization). Of the light beam incident on the polarization beam splitter 103, the S-polarized light component is reflected substantially by total reflection by the inclined surface 103a so as to be radiated towards the light source 101 as the so-called feedback light. On the other hand, of the light beam incident on the polarization beam splitter 103, the P-polarized light is transmitted through the inclined surface 103a substantially by total transmission to exit the polarization beam splitter 103. The light radiated from the polarization beam splitter 103 is converged by the converging lens 106 on the photodetector 107, which then detects the light converged by the converging lens 106 on photoelectric conversion as a reception signal.
Thus, with the communication apparatus shown in FIGS. 37 and 38, employing the polarization beam splitter 103, bidirectional optical communication employing the laser light becomes possible even though no other device is used.
The polarization beam splitting function of the above-described polarization beam splitter is realized by forming a film structure described below on an optical component.
As the technique of adding an optical function, such as the above-mentioned polarization beam splitter function, to an optical element, the operation of optical interference, as occurs when setting the film thickness of a transparent thin film to a value of the number of orders of light wavelength, is frequently used.
It is noted that the condition of interference when the light falls on the sole layer film in a perpendicular direction is shown by the following equation:
nxc3x97d=m(xc2xc)xc3x97xcex
where xcex is the light wavelength, n the refractive index of a monolayer film, m an number of orders of interference and d is a physical film thickness. In general, in the above equation, nxc3x97d is termed the optical film thickness, while the number of orders of interference m is termed the phase thickness of a quarter wave optical thickness (QWOT). For example, in the case of a thin film in which the wavelength xcex of the light used is 550 nm, the refractive index of the monolayer of 2.3 and the physical film thickness d of 59.78 nm, the optical film thickness (nxc3x97d) is 137.5 nm, with the optical film thickness, that is the number of orders of interference m, being 1.
Meanwhile, if a single coating or a monolayer is formed on a substrate as an optical element, there are two boundary surfaces having different refractive indexes between the air and the film, that is a boundary surface between the air and the film (first boundary surface), and a boundary surface between the film and a substrate of the optical element (secondary surface). If, in such case, the phase thickness, that is the number of orders of interference m, is an odd number, phase deviation xcfx80 occurs, so that, as may be seen from the above equation, the reflected waves from the first boundary surface (boundary surface between air and the film) and the reflected waves from the second boundary surface (the boundary surface between the film and the substrate) interfere with each other to give the operation of the reflected waves cancelling each other to a maximum extent. On the other hand, if the phase thickness, that is the value of the number of orders of interference m, is an even number, the phase matching occurs, so that, as may be seen from the above equation, the reflected waves from the first boundary surface (boundary surface between the air and the film) and those from the second boundary surface (boundary surface between the film and the substrate) reinforce each other. Meanwhile, if the value of the phase thickness, that is the number of orders of interference m, is not an integer, there is produced an action lying intermediate between the case where the number of orders of interference is an odd number and that where the number of orders of interference is an even number. Thus, it may be seen that the action of light interference can be controlled by changing the value of the refractive index of the film and the value of the physical film thickness d.
In the above-described polarization beam splitter 103, there is used a thin film structure by multiple layers obtained on alternately layering a thin film of high refractive index and a thin film of low refractive index. The interference operation in the case of the multi-layer film is hereinafter explained.
The optical properties by the action of light interference can be computed by a matrix method employing the optical impedance. For example, if the film refractive index is n, film thickness is d and an angle of incidence of light to the film is xcex8, the characteristic matrix of a transparent monolayer film can be expressed by the following two-row two-column four-terminal matrix:   M  =      [                            m11                          m12                                      m21                          m22                      ]  
where m11, m22 are represented by cosg (m11=m22=cos g), m12 is represented by ixc2x7sing/u (m12=ixc2x7uxc2x7sing) and m21 is represented by ixc2x7uxc2x7sing (m21=ixc2x7uxc2x7sing). On the other hand, g is represented by 2xc2x7xcfx80(nxc2x7dxc2x7cos xcex8)/xcex (g=2xc2x7xcfx80(nxc2x7dxc2x7cos xcex8)/xcex8). For S-polarized light and for P-polarized light, u=nxc2x7cos xcex and u=nxc2x7sec xcex8, respectively.
The characteristic matrix M of a multi-layer film is represented by the product of characteristic matrices M1, M2, . . . , Mi, where i is an integer not less than 1, as indicated by the following equation:
M=(M1)xc3x97(M2)xc3x97. . . xc3x97(Mi).
At this time, the reflectance R of the multi-layer film may be calculated, from the respective elements of the above-mentioned product of the characteristic matrices, the refractive index n0 of an incident medium and a refractive index ns of the substrate, by the following equation:   R  =            "LeftBracketingBar"                                                  (                              m11                +                                  i                  ·                  m12                  ·                  us                                            )                        ·            u0                    -                      (                                          i                ·                m21                            +                              m22                ·                us                                      )                                                              (                              m11                +                                  i                  ·                  m12                  ·                  us                                            )                        ·            u0                    +                      (                                          i                ·                m21                            +                              m22                ·                us                                      )                              "RightBracketingBar"        2  
where u0=n0xc2x7cos xcex80, us=nsxc2x7cos xcex8s for the S-polarized light component and u0=n0xc2x7sec xcex80, us=nsxc2x7sec xcex8s for the S-polarized light component.
The above-described polarization beam splitter 103 may be realized by an alternate layering structure of two sorts of thin-film materials having refractive indices satisfying the so-called Brewster condition.
A more specified illustrative designing of a polarization beam splitter is hereinafter explained.
It is assumed that a polarization beam splitter obtained on bonding two prisms having apex angles of 45xc2x0 is to be designed as a substrate of a polarization beam splitter. Also, in the present embodiment, the desinging wavelength is 780 nm, and a vitreous material for a prism is SF11 (number of optical glass manufactured by SCHOTT INC.). A high refractive index thin film material used is TiO2, with a refractive index of 2.30, whilst a low refractive index material used is SiO2 with a refractive index of 1.46.
Since TiO2 with the refractive index of 2.30 and SiO2 with the refractive index of 1.46 are used as the high refractive index material and the low refractive index material, respectively, as a film combination satisfying the Brewster condition, polarization characteristics satisfying the functions of the polarization beam splitter can be obtained by alternately layering TiO2 and SiO2 as shown below. It is noted that a multi-layer film composed of first to sixteenth layers is formed.
first layer TiO2, d=93.9 nm, nd=216.0 mn
second layer SiO2, d=147.9 mn, nd=215.9 mn
third layer TiO2, d=93.9 nm, nd=216.0 nm
fourth layer SiO2, d=147.9 nm, nd=215.9 nm
fifth layer TiO2, d=93.9 nm, nd=216.0 mn
sixth layer SiO2, d=147.9 nm, nd=215.9 nm
seventh layer TiO2, d=93.9 nm, nd=216.0 nm
eighth layer SiO2, d=147.9 nm, nd=215.9 nm
ninth layer TiO2, d=93.9 nm, nd=216.0 nm
tenth layer SiO2, d=147.9 nm, nd=215.9 nm
eleventh layer TiO2, d=93.9 nm, nd=216.0 nm
twelfth layer SiO2, d=147.9 nm, nd=215.9 nm
thirteenth layer TiO2, d=93.9 nm, nd=216.0 nm
fourteenth layer SiO2, d=147.9 nm, nd=215.9 nm
fifteenth layer TiO2, d=93.9 nm, nd=216.0 nm
sixteenth layer SiO2, d=147.9 nm, nd=215.9 nm
That is, of the first to sixteenth layers, odd-numbered layers, that is the first, third, fifth, seventh, ninth, eleventh, thirteenth and fifteenth layers, are of TiO2, whilst even-numbered layers, that is second, fourth, sixth, eighth, tenth, twelfth, fourteenth and sixteenth layers are of SiO2. Moreover, the physical thickness d of the TiO2 layer as an odd-numbered layer is set to, for example, 93.9 nm, whilst the physical thickness d of the SiO2 layer as an even-numbered layer is set to, for example, 147.9 nm. In addition, an optical film thickness nd of a TiO2 layer as an odd-numbered layer is set to 216.0 nm, with optical film thickness nd of a SiO2 layer as an even-numbered layer is set to 215.9 nm.
Meanwhile, the optical communication apparatus employing the above-described polarization beam splitter suffers from the following problems:
First, the polarization beam splitter is in need of high operational reliability, so that the multi-layered film needs to be fabricated by an expensive electron beam evaporator, whilst there are a large number of film layers and a prism needs to be bonded after formation of the multi-layered film, thus increasing the manufacturing cost considerably.
On the other hand, the polarization beam splitter has such optical characteristics that it has high light incident angle dependency, such that, in order to secure the signal to noise ratio of the communication light (communication signals), it is mandatory to provide a collimator lens for collimating the light beam. Moreover, it is necessary to effect optical axis alignment.
In addition, the conventional optical communication apparatus has a drawback that it has a large number of optical components to render integration difficult.
That is, the conventional optical communication apparatus has a drawback that its manufacturing cost is prohibitive and the apparatus tends to be increased in size.
In view of the above-depicted problem of the prior art, it is an object of the present invention to provide an optical apparatus whereby, if the apparatus is used as an optical communication apparatus, it reduces the cost and size of the apparatus without lowering the communication performance.
The present invention provides an optical apparatus including a main body unit of an optical apparatus, an optical transmission medium connector for connecting the optical transmission medium to the main body unit of the optical apparatus so that an end face of the optical transmission medium is at a pre-set angle with respect to the main body unit of the optical apparatus, a light emitting element fixed in the main body unit of the optical apparatus and adapted for radiating the light, and a sole optical element having a second surface facing the first surface and a connecting surface interconnecting the first and second surfaces. The sole optical element is fixed to the main body unit of the optical apparatus. The first surface has the function of converging a light beam of light incident thereon from outside so that the light beam is focussed at a position spaced a pre-set distance from the first surface. The light emitting element, optical component and the optical transmission medium connector are secured in the main body unit of the optical apparatus in a relative position such that light radiated from the light emitting element is incident on the optical component via the first surface, the light incident on the first surface traverses the inside of the optical component, the light which has traversed the inside of the optical component is reflected on the second surface of the optical component towards the optical transmission medium connector, the light reflected on the second surface is radiated from the coupling surface to outside the optical component, and the light outgoing from the coupling surface is focussed on an end face of the optical transmission medium.
In the optical apparatus according to the present invention, there is also provided a light receiving element at a position lying on the optical axis of the light radiated from the light transmission medium of the main body unit of the optical apparatus.
The optical component is arranged offset from the optical axis of the light radiated from the optical transmission medium.
The optical component is arranged on the optical axis of the light radiated from the optical transmission medium. The light radiated from the optical transmission medium falls on the coupling surface in the optical component via the coupling surface to traverse the inside of the optical component to fall on the light receiving element.
The optical transmission medium connector connects the optical transmission medium at an angle with which the optical axis of the light radiated from the optical transmission medium is inclined with respect to the optical axis direction of the light radiated from the light emitting element to get to the second surface.
The optical transmission medium connector connects the optical transmission medium at an angle such that the optical axis of the light radiated from the optical transmission medium is included in a plane perpendicular to the optical axis direction of light radiated from the light emitting element to get to the second surface of the optical component.
The light receiving element is arranged on the opposite side of the light emitting element with respect to the second surface of the optical component.
The light receiving element is arranged on the side of the light emitting element with respect to the second surface and the light radiated from the optical transmission medium falls on the optical component via the coupling surface to traverse the inside of the optical component to fall on the light receiving element.
The optical component has a diffractive pattern on the first surface.
The optical component further has a third surface which is provided at an area between the second surface and the coupling surface which is at least proximate to the optical transmission medium connector.
The first surface of the optical component is such that the cross-section obtained on slicing the optical component in a plane passing through a first optical axis of light radiated from the light emitting element and getting to the second surface and through a second optical axis of light radiated from the optical transmission medium is convexed towards the light emitting element.
The first surface of the optical component is such that the cross-section obtained on slicing the optical component in a first plane perpendicular to a second plane passing through a first optical axis of light radiated from the light emitting element and getting to the second surface and through a second optical axis of light radiated from the optical transmission medium is convexed towards the light emitting element, with the first plane passing through the first optical axis.
The optical component has a substantially circular cross-sectional shape which is obtained on slicing the optical component in a plane perpendicular to an optical axis of light radiated from the light emitting element and getting to the second surface.
The second surface exhibits total reflection characteristics.